1. Field of the Invention
The present invention relates to an inverter for converting the output of a direct-current power source such as a solar cell, fuel cell, rechargeable cell, or the like into alternating-current power for supply to equipment that needs it, and more particularly to an inverter including a DC/DC converter.
2. Description of the Prior Art
An inverter for a solar light power generation system converts the direct-current (DC) power generated by solar cells into alternating-current (AC) power having a commercial frequency by the use of a DC/AC converting means. The output of the inverter is supplied to an alternating-current load that is connected to a commercial power line so as to be extracted from the commercial power.
FIG. 16 is a block diagram showing an example of a conventional inverter. To the input side of the inverter 4, there are connected solar cell arrays 11, 12, and 13 in parallel, each composed of a plurality of solar cell panels 10 integrally held together in series. The output sides of the solar cell arrays 11, 12, and 13 are connected to a DC/AC conversion circuit 3 so that their outputs are converted into alternating-current power and then output to commercial power 5 by the DC/AC conversion circuit 3.
The solar cell arrays 11, 12, and 13 are installed on the roof of a house or the like, and therefore, depending on the shape and area of the roof, it is difficult to connect equal numbers of solar cell panels 10 in series in all of the solar cell arrays 11, 12, and 13. For this reason, a step-up circuit 2 is provided on the output side of the solar cell array 13 that has fewer cells connected in series so that all the voltages that are fed to the DC/AC conversion circuit 3 are equal. The step-up circuit 2 is incorporated in the inverter 4 as shown in FIG. 16, or is provided separately outside the inverter 4.
In recent years, research has been done on inverters having a DC/DC converter on the input side of a DC/AC conversion circuit 3. Here, the DC/DC converter follows the variation of the amount of sun light received in order to achieve maximum power point tracking control. This permits efficient conversion and output of the power generated.
Also proposed are systems that employ a combination of different types of direct-current power source such as solar cells, fuel sells, rechargeable cells, and the like. Different types of direct-current power source usually output different voltages, and such differences in voltage are generally cancelled by providing a DC/DC converter on the input side of a DC/AC conversion device.
In the inverter 4 configured as described above, when different numbers of cells are connected in series in different branches of the direct-current sources, it is necessary to provide, with reference to the direct-current source that has the largest number of cells connected in series, step-up circuits 2 in the other branches. The step-up circuit 2, however, has a fixed voltage step-up factor, and therefore its output voltage varies as the amount of sun light received varies, resulting in inefficient output.
For this reason, research is being done on inverters having, in each branch of direct-current power sources, a DC/DC converter that each performs maximum power point tracking control, instead of the step-up circuit 2. This configuration, however, makes the inverter large. This problem is encountered also when different types of direct-current power source such as solar cells, fuel sells, rechargeable cells, and the like are used in combination.
A DC/DC converter has a capacitor for rectification on the output side thereof, and outputs a voltage via output terminals respectively conducting to the two poles of the capacitor. The individual output terminals of one pole of a plurality of DC/DC converters are connected together to a common terminal of that pole by way of leads or the like; likewise, the individual output terminals of the other pole are connected together to a common terminal of that pole. The common terminals of the two poles are then connected to the input side of the DC/AC conversion circuit provided in the following stage.
Thus, the distance from the output terminals of one DC/DC converter to the common terminals is inevitably longer than the distance from the output terminals of another DC/DC converter connected in parallel therewith to the common terminals. As a result, the DC/DC converter that has the smaller lead resistance receives a larger voltage and thus has a shorter life time, leading to lower reliability of the inverter. Giving equal lengths to all the leads connecting the output terminals to the common terminals makes wiring complicated and necessitates extra space for wiring. This makes miniaturization of the inverter impossible.
Moreover, connecting the output terminals of a plurality of DC/DC converters to the common terminals by way of leads requires much trouble and time in assembly, and requires space for wiring. This makes miniaturization impossible, and makes the structure complicated. Likewise, connecting the ground terminals of the individual DC/DC converters together to one terminal by way of leads or the like to keep them at the same potential requires much trouble and time in assembly, makes miniaturization impossible, and makes the structure complicated.
Furthermore, arranging the circuit boards of the DC/DC converters in a plane makes the DC/DC converters large and thus makes the miniaturization of the inverter impossible. On the other hand, spatially arranging those circuit boards requires space for wiring between them, and thus likewise makes the miniaturization of the inverter impossible and in addition makes the structure complicated.